1. Field of the Invention
The present invention relates to a 3-phase-to-3-phase power converter and, particularly to a power converter using switching devices controlled in conductive or nonconductive mode.
2. Description of the Prior Art
There has been proposed this kind of a power converter as shown in FIG. 1. The figure shows in a brief block diagram the conventional 3-phase-to-3-phase power converter disclosed in the proceeding of IEEE, Vol. 125, No. 7, July 1978, pp. 643-647, "APPLICATION OF POWER TRANSISTORS TO POLYPHASE REGENERATIVE POWER CONVERTERS". The circuit structure shown in FIG. 1 includes a power source 1 which provides 3-phase (R, S, T) voltages eR, eS and eT, a 3-phase load 2, e.g., a motor, and a main circuit 3 located between the power source 1 and load 2. The main circuit 3 consists of switches 3RU, 3RV, 3RW, 3SU, 3SV, 3SW, 3TU, 3TV, and 3TW that are made conductive selectively so that a 3-phase sinusoidal current is supplied to the load 2. The arrangement further includes a 3-phase current commander 4 which produces a U-phase current command iU*, V-phase current command iV* and W-phase current command iW* for the load 2, three current sensors 5U, 5V and 5W which detect a U-phase current iU, V-phase current iV and W-phase current iW of the load 2, three current controllers 6U, 6V and 6W provided for the U, V and W phases for determining the conductive or nonconductive states of the switches 3RU, 3SU and 3TU; 3RV, 3SV and 3TV; and 3RW, 3SW and 3TW, respectively, a decoder 7 which determines the conductive or nonconductive states of the switches 3RU, 3RV, 3RW; 3RV, 3SU, 3SV; and 3SW, 3TU, 3TV, 3TW in accordance with the voltage condition of the power source 1, and three decoders 8U, 8V and 8W provided for the U, V and W phases for determining the conductive or nonconductive states of the all switches in the main circuit 3 in response to signals 6Ua, 6Va and 6Wa from current controllers 6U, 6V and 6W and a 6-bit signal 7a from the decoder 7.
Next, the operation of the foregoing conventional arrangement will be described. FIG. 2 is a waveform diagram explaining the operation of the decoder 7. The decoder 7 produces the 6-bit signal 7a which identifies 60.degree.-periods I through VI in relationship with the three line voltages eRS, eST and eTR of the power source 1 as shown. The signal 7a is made to have the bit states as shown, causing the switches 3RU, 3RV and 3RW to be nonconductive in periods I and IV, the switches 3TU, 3TV and 3TW to be nonconductive in periods II and V, and the switches 3SU, 3SV and 3SW to be nonconductive in periods III and VI. During this operation, the main circuit 3 has the respective conductive states as shown equivalently in FIGS. 3(I) through 3(VI). These equivalent circuits of the main circuit 3 are given the positive polarity for the upper input line and the negative polarity for the lower input line, and the voltage between the lines is a maximum instantaneous value of the absolute value of the three line voltages of the power source 1. Accordingly, the decoder 7 operates on the main circuit 3 to function as a 3-phase inverter that produces a d.c. voltage having a maximum instantaneous value of the absolute value of the three line voltages of the power source 1.
The current commander 4, current sensors 5U, 5V and 5W, and current controllers 6U, 6V and 6W operate to control the current of the load 2 by making the switches 3RU through 3TW conductive or nonconductive in the same way as in the usual inverter current control. For example, the current commander 4 produces the current commands iU*, iV* and iW* for 3-phase sinusoidal waves with a variable magnitude and frequency, and the current controllers 6U, 6V and 6W produce the signals 6Ua, 6Va and 6Wa for making the switches conductive during the periods depending on the differences between iU* and iU, between iV* and iV, and between iW* and iW. The signals 6Ua, 6Va and 6Wa are received by the corresponding decoders 8U, 8V and 8W, which also receive the signal 7a from the decoder 7 to cause the switches in the main circuit 3 to be conductive or nonconductive according to these input signals, so that power is supplied from the power source 1 to the load 2.
The conventional 3-phase-to-3-phase power converter arranged as described above does not control the current waveform of the 3-phase power source, resulting in the disorder of a current flow in the 60.degree.-period in some current waveform conditions, and therefore it does not provide a satisfactory input power factor.